1. Field of the Invention
The present invention relates to an apparatus for controlling combustion in engines, and more particularly, to an apparatus for controlling the intake air amount when performing lean combustion in engines.
2. Description of the Related Art
In a typical engine, fuel is injected toward an intake port from a fuel injection valve. The fuel is uniformly mixed with air and then sent into the combustion chambers. In such an engine, an air intake passage is opened and closed by a throttle valve, which is operated in accordance with the manipulation of an acceleration pedal. The movement of the throttle valve adjusts the amount of air that is drawn into the intake port to be uniformly mixed with the fuel. The air-fuel mixture is then sent into the combustion chamber. The engine power is controlled in this manner.
The above engine is a type of engine that performs so-called uniform charge combustion. In such type of engine, the closing action of the throttle valve in the intake passage produces a large negative pressure and increases pumping loss. To cope with such problems, engines that perform so-called stratified charge combustion have been proposed. When performing stratified charge combustion, the throttle valve is widely opened and the fuel is directly injected into each combustion chamber. This causes the air-fuel mixture concentrated about the ignition plug to have a low air-fuel ratio, which enhances the ignitability of the mixture at this part of the combustion chamber.
When the engine load is small, stratified charge combustion is performed by concentrating the injected fuel about the ignition plug and completely opening the throttle valve. This increases fuel efficiency and decreases pumping loss. When the engine load is great, the throttle valve is opened and closed in accordance with the depressed amount of the acceleration pedal. Furthermore, the amount of injected fuel is feedback controlled.
There is also a type of engine that performs lean combustion. Japanese Unexamined Patent Publication No. 5-18303 describes lean combustion. When performing lean combustion, the ratio of fuel in the mixture is decreased. During acceleration or under similar conditions, the ratio of fuel is increased to perform combustion at a stoichiometric air-fuel ratio. The stoichiometric air-fuel ratio is feedback controlled.
In each of the above types of combustion, changes in the operating state of the engine (for example, when the engine load increases) causes the engine to switch from stratified charge combustion to uniform charge combustion, or from lean control to stoichiometric control, as shown in FIG. 9. When performing uniform charge combustion or stoichiometric control, the amount of fuel injection is feedback controlled so that the air-fuel ratio becomes stoichiometric.
However, if the amount of intake air fluctuates, for example, when the throttle valve becomes clogged or when the dimensions of the tappet clearance changes, the amount of the injected fuel must be corrected accordingly to maintain the stoichiometric air-fuel ratio. Under such conditions, the value of the corrected fuel injection amount is learned by a computer. Thus, when the engine switches from stratified charge combustion to uniform charge combustion or from lean control to stoichiometric control, the amount of injected fuel is controlled in accordance with the learned value. This may cause a sudden change in the amount of injected fuel and may produce a torque shock.